This invention relates to a n image forming apparatus, and in particular, to an image forming apparatus in which an image of an original document is read by photoelectric conversion, the image data read by photoelectric conversion are once memorized and kept to be read out selectively, and the image forming on both sides of a recording member can be made on the basis of said read out image data.
It is known heretofore a digital copying machine which has a structure such that an original document is scanned optically to obtain an optical image, which is received and converted by a photoelectric conversion element such as a line image sensor into an electrical signal, the electrical signal further digitalized, the digitalized image signal is then subjected to the image processing such as re-sizing processing, filter processing, and .gamma. correction processing, and a latent electrostatic image is formed on a photoreceptor by a laser beam modulated on the basis of said image-processed image signal (TOKKAISHOU 62-157070).
In the meantime, the images which are supposed to be the original in the above-mentioned digital copying machine, are of many kinds and varieties, and the typical ones are a letter image, a photographic image, and a mixed image of both the letter and photograph type. Accordingly, on the occasion of every kind of image processing for the read image information, the kind of the image is discriminated and the image processing having the characteristics in accordance with the kind according to the discrimination result is carried out.
Further, in the usual image forming apparatus capable of forming images on both sides of a transfer member, the toner images are formed on its both sides by repeating the process of fixing the image on its one side, hence the image processing corresponding to the front or reverse side is made in the same manner regardless of whether the side is front or reverse.
Now, the digital copying machine capable of double-sided copying related to this invention is one such that the toner images are formed on both sides of the transfer member, then fixed together at the same time, and the series of operations consisting of reading, image discrimination, image processing, image forming on both sides of the transfer member (printing) are usually carried out in real time, further in the case where plural copies are made from an original, each original page is read the same number of times as the number of copies and the image processing as well as printing operation is repeated for every reading operation.
Accordingly, in the case where plural copies are made from an original of plural pages by repeating the process of making one copy for each page, the method such that the original sheet for which image reading is once finished is recirculated to the image reading portion by an automatic document feeder (ADF) is employed. The apparatus for recirculating the original sheets to make one copy for each of them, so that one set of copies may be completed followed by another, is generally called a recirculating document handler (RDH).
In this case, where plural sets of copies are made from an original, it is inevitable to repeat the copying operation the number of times of copies, but repeated reading of the same original pages for each copying cycle hinders a high-speed processing, and in particular, in the case of operating the recirculating document handler (RDH), the time for copying is increased by the transporting operation for the original sheets to be done for each reading process, and further the structure of the recirculating document handler becomes complicated.
On the other hand, with recent reductions in the cost memory circuits, it has become possible to actualize the copying machine provided with a memory circuit for memorizing the image data. Then, by making said memory circuit memorize and output collectively the read data of the plural images of the original, the inefficient repeating of reading operation for each of the original sheets can be ommitted, and in particular, in the case where plural copies are made from an original of plural pages by repeating the process of making one copy for each page, a high-speed processing becomes possible.
On the other hand, because the number of times of executing a transfer process for the reverse side image is different from that for the front side, it is necessary that image processing should be made in a manner such that the print qualities of both the front and reverse sides are not different from each other.
Further, if the memory circuit is made up in such a manner as to be divided into page units (image units), it raises a problem that the memory capacity can not be utilized effectively because of many useless memory portions, although it makes reading-out of the image data easy. Still further, in some cases, copies are desired to be made from a double-sided original, for example, of A4 size to one side of A4 size of copy paper sheet, or from a single-sided original of A4 size to both sides of copy paper sheet of A4 size, and further, for example, images on a double-sided original of A4 size are desired to be copied onto the one side of A3 sized copy paper sheet in a side-by-side manner. Then, in the method of making the image data of the plural page original to be memorized in said memory circuit, it is desired that the processing concerning whether the image is for the front side or the reverse side and the processing for the development of images of A4 size to A3 size as the above-mentioned can be carried out easily. In other words, if the memorized image data are already specified as those for the front side or for the reverse side, it will raise a problem on the occasion of changing the mode of original input and copy output.
U.S. patent application Ser. No. 08/821,878 (now U.S. Pat. No. 5,761,573 issued Jun. 2, 1998) teaches an image forming apparatus in which a processing condition or an image data correcting method for a reverse side toner image is different from those for an obverse side toner image, the reverse side toner image and the obverse side toner image are formed on a reverse side and an obverse side of a sheet and fixed simultaneously. However, this application teaches nothing about a technique in which a document image is read by an image reading apparatus, after image signals are stored in a memory, image data are read from the memory and subject to image correction processing for a reverse side toner image and image correction processing for an obverse side toner image separately, and then the reverse side toner image and the obverse side toner image are formed.
If the image data for a reverse side toner image and the image data for an obverse side toner image are stored in the memory after original image were converted, it may be difficult to follow a change in output format such as a change between a single side image formation mode and a both side image formation mode. Further, if all image data processing are separated for a reverse side toner image and for an obverse side toner image, a working load on a CPU to do the image data processing becomes too heavy and a control for them becomes complicated.
As is described in the foregoing, the filter processing and .gamma. correction are generally executed with different characteristics depending on the kind of the original, while in the facsimile apparatus the method of data compression and compression rate are made to be changed for each kind of originals.
Further, on this occasion, because the number of times of executing a transfer process for the reverse side image is different from that for the front side, it is necessary that image processing should be made in a manner such that the print qualities of both the front and reverse sides are not different from each other.
For this reason, there has been a problem that the image memory circuit is made to be complicated, because, in the case where a digital copying machine is provided with a memory circuit for memorizing the compression-processed image data in said memory circuit, if the aforesaid compression processing technology depending on the kind of the image is employed as it is, it becomes necessary that such sets of information as the information on discrimination of the images, the information on the method of compression applied, and the information on the division of the image area for the images with mixed presence of photographs and letters are memorized as well as the image data.
On the occasion of memorizing the image data in the memory circuit, it is required to execute the image compression processing in order to utilize the memory capacity efficiently, but in some cases the user has a preference for an increase of the number of original pages to be memorized over the image quality, and some other cases the user has preference for a high image quality to be obtained over the number of pages to be memorized, thus it has been desired to provide an apparatus capable of coping with such difference in the requirements.
Further, there has been also a problem that if the whole area of the image to be read is made the object of reading regardless of the original size and the recording paper size, it means that the image data of the blank spaces which are not necessary to be memorized and the image data not to be used in the image forming are memorized uselessly, resulting in the waste of the memory capacity.
In the meantime, some digital copying machines have a function to make image forming on the basis of the image data inputted from an external apparatus (function as a general printer), and in the case where a copying machine has such a function as mentioned above, it generally comprises a buffer memory in order to adjust the timing relationship between the image data input from the external apparatus and the image forming in the copying machine. However, it is a problem that the circuit structure becomes complicated for the reason of having separate memory circuits for every function if it has separately a buffer memory for memorizing the image data in a similar manner in addition to the memory circuit for memorizing and storing the image data as mentioned above.